1. Field of the Invention
The field of the invention is that of composite materials based on conductive polymers, which can be used for coatings that meet goals of achieving electromagnetic wide-band attenuation.
To attain goals such as these, it is necessary to control the dielectrical properties of the materials used so as to superimpose layers of materials, the properties of which give the resulting structure an impedance gradient.
2. Description of the Prior Art
At present, materials based on conductive polymer show promising characteristics for the absorption of electromagnetic waves. However, the control of the conductive properties with the conductive polymer content in the material is still very uncertain owing to the phenomenon of percolation.
More specifically, this phenomenon of physics, which corresponds to the sudden passing from the properties of an insulator substance A to those of a conductive substance B when the populations A and B are mixed, depends on various parameters such as the size and shape of the conductive particle and the nature of the insulating binder matrix (its compatibility and its wetting capacity). Thus it has been shown by B. Wessling, at the ANTEC Congress 91, Montreal, Canada, that the phenomenon of percolation occurs at between 1% and 5% by weight in the case of polyaniline dispersed in a polyvinyl chloride (PVC) matrix or nylon matrix making the conductivity go suddenly from about 10.sup.-4 S/cm to 1 S/cm. Other work by B. Tieke (CIBA GEIGY, Polymer 1990, 31, 20) shows that polypyrrole electropolymerized in a polyimide has a percolation jump that is even lower between 0.2% and 0.5% by weight. With composite materials such as these, it is therefore not possible to control the conductivity as a function of the conductive charge content since there is a sudden change from an insulator state to a conductive state, the mechanical properties of which are those of the conductive polymer giving rise to brittle materials that are hard to use and .do not benefit from the filmogenic properties of the binder matrix.